Cycles Shadow Alpha Render

Hi, since long time i am looking for solution for my problem. I search on forum but my problem is more spesific.
http://img831.imageshack.us/img831/3310/54958927316357945066816.jpg

-This image taken from Blender Internal Render Engine.
-Background has alpha channel and i want to take this render with cycles too.
-I add a plane and check on “only shadow” button in Blender Internal Material Settings.
-The question is how can i take render in Cycles with shadows.

Thanks in advance…

any help guys?

63 view and no answer or suggestions?

Thanks stusee,

what if you don’t want to composite it onto a background image or movie, but just want the final render to have an alpha background? I tried a RenderLayer with nothing in it instead of the Image node but that didn’t work. Is there another way to do it?

Thanks in advance.